The thermal resistance of the exterior setting which surrounds energy cables has the biggest influence on the temperature rise of the conductor and therefore the present ranking in contrast with different components. That’s the reason loads of effort is taken by asset homeowners and cable methods designers to scale back the exterior thermal resistance for cables to maximise their current-carrying capability.
It is not uncommon follow to put in buried teams of low- or medium voltage cables inside duct banks which permits circuits to be well-organised and guarded in the identical trench (or a number of trenches). Concrete is usually used to encompass the cables in a duct financial institution and has comparatively low thermal resistivity but additionally will not be topic to moisture migration (drying-out impact), which ensures larger cable present scores.
Excessive-voltage cables are generally buried in trenches and surrounded with an authorized bedding materials (sometimes thermally stabilised sand) which has appropriate low thermal resistivity. Backfilling is only when the thermal resistivity of the native soil is excessive but additionally will be an efficient methodology of avoiding moisture migration (resulting in excessive thermal resistivities) within the neighborhood of the cables. A number of transmission cable circuits could also be put in in separate backfills throughout the similar transmission easement. Joint bays made from precast concrete sections or manufactured on-site and will be unfilled or crammed with stabilised sand.
As soon as cables are put in in a duct financial institution or a backfilled trench the remaining area left is stuffed both with the native soil (usually sieved for rocks) or a unique materials introduced in from off-site.
Probably the most broadly used paperwork for cable present ranking calculations are the IEC 60287 [ref. 1] sequence of requirements (which adopts an equal thermal mannequin solved utilizing analytical strategies). Whereas most real-world circumstances together with these described above contain a number of zones with completely different thermal resistivity, the IEC normal offers a calculation methodology to permit for the presence of a single a duct financial institution or backfill solely and the scale are additionally restricted.
For correct present ranking calculations of cables put in in multi-layered or multi-zoned soils with various thermal resistivity the Finite Aspect Technique (FEM) is confirmed to be extremely efficient.
The IEC 60287 and FEM calculations herein had been carried out utilizing ELEK Cable HV Software program™.